Hello, a student of mine brought up a great
question. Why is grass under snow still green, but
grass under a rock yellow? I was able to answer
the part about the rock, but wasn't able to give a
complete answer about the snow. Thanks!

Question Date: 2018-03-16

Answer 1:

I agree, that’s a great question. You’re doing a
great job if students are asking questions like
that! I’m not a plant physiologist, but here’s my
guess about what’s going on. It sounds like you
already explained that when a plant isn’t
getting light, it stops making chlorophyll.
Chlorophyll breaks down, so the yellow
(xanthophyll) and orange (carotene) pigments are
more visible.

The pigment breakdown happens even faster in
sunlight (just like cloth will fade in the sun).
That explains why we have to move the lawn
furniture around.

Sometimes the grass gets all brown and dry in
the fall, but it depends on your climate and
the type of grass. This is senescence,
where the top of the plant seems to die, but it’s
really more like the leaves falling off a tree.

The underground part of the grass plant isn’t
dead at all. So what triggers senescence? Daylength and temperature, plus the genetics of
the plant.

The plant stops growing well before the first
freeze. A “killing freeze” or “hard freeze” (25
Fahrenheit or -4 Centigrade for at least 4 hours)
will not really kill the plant, but will destroy
cells above the ground. Ice crystals poke through
the cell walls, killing the cells, which will then
dry out and turn brown. A frost (with the
temperatures just below freezing) doesn’t damage
the cells unless someone walks on the frosty
grass.

My guess is that it’s all about timing.
Say the fall has been reasonably sunny and not too
cold. There may have been a frost, but not a
killing freeze, so senescence hasn’t happened.
Then the snow falls, covering the green grass.
Now the plant isn’t making more chlorophyll, but
the light is also not breaking it down. Freezing
may even help to preserve the chlorophyll that’s
there, just like spinach in the freezer (total
guess). Snow is a good insulator, so maybe the
snow-covered grass didn’t even get cold enough to
break down the cells and the grass will not even
turn yellow after it has thawed out.

I hope this is helpful. It might give you some
experiments to try with your kids. Let me know
what you find out!

Thanks for asking,

Answer 2:

It's because the grasses that live in
environments that get snow are merely dormant, not
actually dead, while the grasses under a rock
can't get light and so die. Also, it's usually
only brief snows that have green grass under them
- if it's a long snow season, those grasses will
dry out and die, too (although the roots
underneath the ground are still alive, and will
put up more green leaves when the snow melts).

Answer 3:

I haven't noticed whether or not grass is green
under the snow. If it's green, it's getting
enough light [and heat?] to carry out
photosynthesis.

Here's a poem about green grass under the snow,
but that's not science, so I looked farther.

The link below says that for the most part
grass will survive its winter die back under the
snow. However, a blanket of snow or coat of frost
does make a lawn more susceptible to smother or
trample damage. Anywhere toys, tools, sprinklers
or whatever other debris is left on a lawn over
the winter, one can expect to find a brown spot
beneath. Be sure to have a lawn clean and clear
before the snow falls, or as soon after as
possible. The same goes for trample damage- your
lawn is sensitive to trails being stomped out
during the winter, so keep the on-lawn traffic to
a minimum.
read here

There are a few things I can think of, but I do
not think this is a complete answer, just
references. Rocks tend to block both moisture and
sunlight, so grass under a rock would probably
grow poorly because it lacks two of the most
essential plant nutrients. However, snow would
at least provide some moisture to the grass
underneath, and it may also provide some
insulation from heat loss. My hypothesis is
that grass under snow is "watered" by the moisture
from the snow and partially protected from the
cold air by the insulation.